1. Field of the Invention
The present invention relates to an instrument for drilling dental root canals.
2. Description of the Related Art
The treatment of an infected dental root is effected by extracting the pulp with the aid of appropriate instruments, then by shaping the root canal by successive drilling operations, traditionally carried out with the aid of instruments of varying size and conicity. The final operation consists of filling the root canal.
The shaping of the root canal consists initially of enlarging the canal in its crown and median parts in order subsequently to permit easier treatment of the apical part of the canal by mechanical cleaning of the infected tissues.
An instrument designed for drilling dental root canals generally has a tapering rod fitted into a handle to permit it to be moved manually or mechanically and having helical flutes on at least a part of its length, which flutes have a pitch which may or may not be constant, and having at least one cutting edge.
When being used in continuous rotation, this type of instrument can tend to screw itself into the canal. EP 0 801 930 describes a dental reamer enabling this screwing problem to be avoided and being characterised in that the tangent to the cutting point of the instrument tends to be superimposed with the tangent to the diameter of the circle in which a cutting cross-section is inscribed and passing via the said cutting point.
U.S. 2005/0282108 describes an endodontic file including a file body extending from a tip region to a shank region and at least one helical flute having a cutting edge and extending from the tip region towards the shank region. In one embodiment, the file according is characterised by the fact that the clearance angle defined by the cutting edge varies between the tip region and the shank region of the file and in particular decreases from the shank region to the tip region. The smaller the clearance angle the greater the forces, and especially the friction forces exerted on the file. However, in the disclosed embodiment, the decreasing of the clearance angle leads to a decreasing of the convexity of the convex portion of the flute which in turn leads to a loss of strength of the file. This could therefore lead to a file having reduced strength especially in the tip portion. In another embodiment, the file is characterised in that the flute depth measuring the deepest portion of the flute is not constant along the entire working portion of the endodontic file, but rather decreases from the shank portion to the tip portion. The decrease of the flute depth allows reinforcing the file at its point where it is the most fragile and where the forces and torsional stress are the greatest. However, the decrease of the flute depth leads to a decrease in the effectiveness of the cutting edge, particularly at the tip. Thus, with this embodiment, a file with improved strength but less effectiveness is obtained.
Apart from the screwing problem, another problem arising in the production of instruments for drilling root canals is that of the strength and flexibility of the instruments. In fact, when the instrument is too flexible it may bend over or break before the practitioner has been able to terminate the operation, and when the instrument is too rigid, it follows the curvature of the dental root canal only with difficulty.
Similarly the effectiveness of a cutting edge is assessed at its cutting angle. When considering a cross-section of an instrument inscribed in a circumscribed circle passing via its cutting edges, the cutting angle of a cutting edge of the instrument is defined as the angle between a diameter of the circumscribed circle of the cross-section passing via the cutting edge and the tangent to the side of the cross-section having as its vertex the said cutting edge (the said side is determined according to the direction of rotation of the helical flutes of the instrument). The more acute the cutting angle of a cutting edge (even negatively), the greater the effectiveness of the said edge. However, in general the more acute the cutting angle, the more acute is the angle at the vertex of a cutting edge, i.e. the finer and more fragile the cutting edge, which means that the instrument which becomes thinner towards its point also becomes more fragile. However, it is at the point of the instrument that the forces and torsion are the greatest. It is therefore also necessary to take into account the effectiveness of an instrument and to find a compromise between effectiveness, flexibility and strength.